A conventional diaphragm for a speaker is known such as in patent literatures 1 and 2. A description is made of a conventional speaker diaphragm using FIG. 6. FIG. 6 is a sectional view of a speaker diaphragm made of resin by conventional injection molding.
As shown in FIG. 6, conventional speaker diaphragm 31 is produced by heat-dissolving resin pellets (e.g. polypropylene) and injection molding into a preliminarily shaped mold.
A typical example of a resin material for injection molding is a single material such as polypropylene. Besides, in order to adjust physical properties as a speaker diaphragm (i.e. to adjust speaker characteristics and sound quality), a blended material (different resins are used) is used.
Further, to adjust other physical properties difficult to adjust by the resins, a reinforcing material such as mica is mixed to adjust speaker characteristics and sound quality. To further increase the degree of flexibility in adjusting physical properties, a pulp material is mixed for sound quality adjustment.
As to the latest audio and video devices and speaker-mounted device (e.g. an automobile) with these electronic devices on board, their performance has been dramatically improved as compared to conventional ones owing to the significant progress of digital technologies.
As to sound quality, reality has been increased by lower distortion, a wider bandwidth, and a higher dynamic range. As to image quality, its performance has been remarkably improved by finer resolution and the advent and proliferation of large-size modules (e.g. plasma display panel).
Accordingly, with the performance improvement of the above electronic devices, the market strongly demands performance improvement from speakers used for the electronic devices as well.
Hence, a speaker in such a situation essentially requires higher performance of the speaker diaphragm, a major factor in determining sound quality among the speaker components. Speaker diaphragms, however, are typically made of paper or resin because they are produced by traditional papermaking, or injection molding or press working of resin.
Under the circumstances, these speaker diaphragms have been used according to their uses while taking advantage of each feature. However, they have their respective disadvantages and do not satisfy the above-described market request.
Specifically, with a paper diaphragm, physical properties of the speaker diaphragm can be set minutely, which allows increasing the degree of flexibility in adjusting characteristics as a speaker and sound quality. The diaphragm, however, has low moisture-proof reliability and strength, which are disadvantages specific to paper. Further, producing the diaphragm requires papermaking, namely a large number of production steps.
A resin diaphragm, meanwhile, secures moisture-proof reliability and strength; has a good appearance; and increases productivity. The diaphragm, however, can secure only uniform physical properties specific to resin, which is inevitable. Hence, the diaphragm is disadvantageous in its extremely small range of adjusting characteristics as a speaker and sound quality.
A speaker diaphragm produced by mixing resin and pulp material has a large degree of flexibility in adjusting sound quality and secures moisture-proof reliability. To improve physical properties and sound quality, however, the strength of the speaker diaphragm needs to be increased.    [Patent literature 1] Japanese Utility Model Unexamined Publication No. H03-56287    [Patent literature 2] Japanese Patent Unexamined Publication No. 2003-204588